I am currently adding a picture window to an existing gable end wall. This gable end wall is part of a room with a cathedral/vaulted ceiling. I know how to frame it with king studs, a header, jacks, sill, cripples/etc. However, I'm not sure how to support the ceiling above since it's on a pitch while framing. Does the gable end NEED a temporary support wall and if so, how would you build one in this space?
Assuming a modern engineered (scissor) truss system, you do not need to support the ceiling. You have common trusses running across the room to support the ceiling, and there's a gable truss over the wall which will carry the outer roof. You can cut your opening without concern of settling. The wall as a system, even with just fiber sheathing, will be stable.
A year ago I would have said that the gable end is always non-load bearing, so don't worry about it. This guy, however, had a load bearing gable end with vaulted ceilings like yours. You should knock on the ceiling along the gable end and perpendicular to the gable end to verify that the rafters run parallel to the gable end. What you're looking for is rafters parallel to the gable end with a rafter located within 24" of the gable end. That implies that your wall is non-load bearing.
See IRC R602.7.4 for the header size in a non-load bearing wall. Just a flat 2x4.
